Interruption of the Ras/MEK/ERK signaling cascade enhances Chk1 inhibitor–induced DNA damage in vitro and in vivo in human multiple myeloma cells

Abstract

IntroductionCheckpoint kinases (ie, Chk1 and Chk2) represent key components of the DNA damage checkpoint machinery, which monitors DNA breaks caused by endogenous/metabolic or environmental genotoxic insults or by replication stress. 1,2 In response to DNA damage, cells activate checkpoint pathways, resulting in cell-cycle arrest, which permits the DNA repair machinery to rectify the damage. Depending on the nature of the DNA lesions and the context in which damage occurs, cells either survive and resume cell-cycle progression through a recovery mechanism when repair is successful or are eliminated by apoptosis if repair fails. Thus, checkpoints provide normal cells with critical surveillance machinery designed to promote genomic integrity and survival. Conversely, checkpoint dysfunction contributes to tumorigenesis by permitting cell proliferation in the face of genomic instability. 3,4 Moreover, checkpoints are activated by numerous chemotherapeutic agents and ionizing radiation. 5 This has prompted the development of anticancer strategies targeting checkpoint machinery. 5,6 Among the diverse checkpoint pathway components, Chk1 represents a particularly attractive target for several reasons, that is, (1) Chk1 is functionally associated with all known checkpoints (eg, the G2-M transition, G1, intra-S, 5 and, most recently, the mitotic spindle checkpoint 7 ); (2) Chk1 is essential for maintenance of genomic integrity, whereas the role of Chk2 is conditional 3 ; and (3) for multiple checkpoints, Chk2 function can be mimicked by Chk1, whereas Chk1 cannot be replaced by a functionally overlapping kinase such as Chk2. 3 Chk1 inhibition (eg, by the Chk1 inhibitor UCN-01) results in abrogation of checkpoints induced by DNA-damaging chemotherapy and radiation, leading to enhanced tumor cell killing. 8,9 Given these findings, a major emphasis has been placed on efforts to combine Chk1 inhibitors (eg, UCN-01 10 or CHIR-124 11 ) with diverse DNA-damaging agents. However, an alternative strategy is based on the concept that transformed cells may be ill-equipped to survive simultaneous interruption of both checkpoint machinery and prosurvival signaling. In this context, our group has reported that exposure of human leukemia and multiple myeloma (MM) cells to UCN-01 induces pronounced activation of MEK1/2 and ERK1/2, 12,13 key components of the Ras/Raf/MEK/ERK cascade that plays a critical role in proliferation and survival of malignant cells. 14 Significantly, disruption of ERK1/2 activation by pharmacologic MEK1/2 inhibitors, 12,13 farnesyltransferase inhibitors (FTIs; eg, L744832) 15,16 or HMG-CoA reductase inhibitors (ie, statins) 17 results in a dramatic increase in apoptosis of hematopoietic malignant cells. Together, these findings suggest that activation of Ras/MEK/ERK signaling cascade may represent a compensatory response to Chk1 inhibitor lethality, and that interruption of this response lowers the death threshold.Although the observation that MEK1/2 inhibitors or FTIs antagonize UCN-01-mediated ERK1/2 activati...

Differential response of normal and malignant urothelial cells to CHK1 and ATM inhibitors

“…Hence, it is not surprising that aberrant regulation of MAPK cascades contribute to cancer and other human diseases. Interestingly, exposure of various multiple myeloma cells to non-cytotoxic concentrations of Chk1 inhibitors induced DNA damage and a little cell death accompanied by Ras and ERK1/2 activation (Dai et al, 2008;Dai et al, 2001). Furthermore, interruption of the MAPK signalling cascade also enhanced Chk1 inhibitor-induced DNA damage and apoptosis both in vitro and in vivo (Dai et al, 2008).…”

“…Interestingly, exposure of various multiple myeloma cells to non-cytotoxic concentrations of Chk1 inhibitors induced DNA damage and a little cell death accompanied by Ras and ERK1/2 activation (Dai et al, 2008;Dai et al, 2001). Furthermore, interruption of the MAPK signalling cascade also enhanced Chk1 inhibitor-induced DNA damage and apoptosis both in vitro and in vivo (Dai et al, 2008). Deregulated Ras signalling was found to compromise DNA damage checkpoint recovery after completing DNA repair in S. cerevisiae yeast.…”

microRNA 28 controls cell proliferation and is down-regulated in B-cell lymphomas

“…Deregulation of ERK signaling is found in many different cancers, including B-NHL, and its inhibition is also associated with an increase in apoptosis (31,32), including MYC-induced apoptosis (28). Thus, elevated BAG1 levels can protect the cells from apoptosis and, by activating ERK1/2, may favor survival and proliferation in lymphomas lacking miR-28 expression.…”

A mechanistic rationale for MEK inhibitor therapy in myeloma based on blockade of MAF oncogene expression

“…Previous preclinical studies have documented activity of MEK inhibitors against myeloma cell lines and osteoclasts. [8][9][10] A novel, potent inhibitor of MEK1/2 affected MAF protein levels in the MM cell line H929. 11 We now extend these findings to a larger panel of MM cell lines to investigate whether particular myeloma subtypes would be especially sensitive to MEK inhibition.…”